1. Field of the Invention
The present invention relates to an image forming apparatus having a function of measuring a color of a measurement image and a control method of the image forming apparatus.
2. Description of the Related Art
Image quality of an image forming apparatus includes parameters such as granularity, in-plane uniformity, character quality, color reproducibility (including color stability), and the like. Multi-color image forming apparatuses have become popular these days, and there is an opinion that the most important image quality parameter may be color reproducibility.
People have expectations of colors for objects (especially for human skin, blue sky, metal, or the like) based on their memories and experiences, and thus if a color of an object is different from their expectations beyond a tolerance level, they have feelings of strangeness. Such colors are referred to as memory colors, and image forming apparatuses are often demanded to provide the high reproducibility of the color when outputting photographs or the like.
A degree of demand for the color reproducibility (including stability) by the image forming apparatus has increased among office users who have the feelings of strangeness caused by color difference between an image on a monitor and a document image or a photographic image, and among graphic art users who pursue the color reproducibility of computer graphic (CG) images.
In such a situation, for example, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus that reads a measurement image (patch image) formed on a sheet by a measurement unit (color sensor) disposed on a conveyance path of the sheet, to satisfy a demand for the color reproducibility by users. According to the image forming apparatus, based on a reading result of the patch image by the color sensor, feedback is given to process conditions including an exposure amount and development bypass to reproduce density, gradation, and color at a certain level.
However, in a color sensor according to Japanese Patent Application Laid-Open No. 2004-086013, accuracy for detecting a color is deteriorated due to factors such as an output fluctuation of a light source caused by a change of an environment temperature. Thus, it is considered that a white reference plate is disposed at a position facing to the color sensor, and the white reference plate is measured by the color sensor to correct a detection value of the color sensor.
More specifically, when a reflected light from a white reference plate is defined as W(λ) and a reflected light from a patch image is defined as P(λ), a spectral reflectance R(λ) of the patch image can be acquired by the following equation.R(λ)=P(λ)/W(λ)  (Equation 1)
When the spectral reflectance of the patch image is acquired using the white reference plate, the white reference plate needs to be arranged at a position near a measurement surface of the color sensor to perform measurement. However, when the patch image is measured, the sheet on which the patch image is formed may hit the white reference plate to cause a jam. Thus, a movement mechanism may be provided for the white reference plate to move the white reference plate closer to the measurement surface of the color sensor when the white reference plate is measured and to move the white reference plate away from the measurement surface of the color sensor when the patch image is measured so that the sheet does not hit the white reference plate.
However, the movement of the white reference plate may deteriorate a positional accuracy and generate a deviation in a distance from the color sensor to the white reference plate. Accordingly, an amount of the reflected light from the white reference plate fluctuates, and thus a detection value of the color sensor cannot be accurately corrected.